Flexible thermoplastic olefin blends are used extensively in commercial applications, such as roofing membranes and geomembranes, automotive interior skins, and wall coverings. These applications typically require an excellent balance of flexibility, tensile strength, extensibility, self-hot tack, tear and puncture resistance, heat resistance, and low temperature flexibility.
Flexible thermoplastic olefin blends are typically a blend of a polypropylene with an elastomer, such as, ethylene-propylene rubber (EPR), ethylene-propylene-diene rubber (EPDM), elastomeric propylene-alpha olefin copolymers, and elastomeric ethylene-alpha olefin copolymers. The polypropylene typically provides tensile strength and heat resistance to the blend, while the elastomer provides flexibility, extensibility and good self-hot tack.
PCT publication WO 00/69965 describes thermoplastic filled membranes utilizing propylene copolymers in combination with polypropylene. The propylene copolymers disclosed in this reference have both narrow composition distributions (i.e. uniform composition distribution) and narrow molecular weight distribution. It is believed that the use of propylene copolymers having narrow composition distribution limits the heat sealability window of the blend and reduces the compatibility between the polypropylene component and the elastomer component. For purposes of this invention, the heat sealability window is described as the temperature range within which the blend will exhibit an adequate seal initiation temperature, while at the same time maintaining sufficient self-hot tack strength.
The seal initiation temperature can be lowered by increasing the relative amount of propylene-alpha olefin copolymer in the blend and/or by adding additional components such as process oils. However, these type of changes will also reduce the heat resistance, tensile strength and hot tack performance of articles made from the blend.
What is desired is a flexible thermoplastic olefin blend that exhibits an excellent balance of flexibility, tensile strength, extensibility, and has a broad heat sealability window and good heat resistance. Surprisingly, the inventors have found that a flexible thermoplastic olefin blend of (a) an isotactic polypropylene, and (b) a propylene-alpha olefin copolymer having substantially isotactic propylene sequences and exhibiting a narrow molecular weight distribution provides better physical properties, such as puncture resistance and tear, higher ultimate seal strength and tensile strength; and a broad composition distribution provides improved compatibility between the blend components, which results in an excellent balance of flexibility, heat resistance, extensibility, tensile strength, tear resistance and improved heat sealability window. It is further believed that, the use of a propylene-alpha olefin copolymer in accordance with the invention (which has a broad composition distribution in addition to a narrow MWD) allows for the development of high performance blends encompassing greater formulation flexibility and property balance with respect the compositional range of polypropylene and propylene-alpha olefin copolymers utilized.